1. Field
This invention relates generally to automatic gain control (AGC). More specifically, this invention relates to a system and method for automatically controlling gain in a signal chain.
2. General Background and Related Art
In many signal processing applications, an input signal to a signal chain may include both desired and undesired signal components. The magnitude of a desired signal may be too small. Conversely, the magnitude of an undesired signal may be too large. Processing mechanisms, such as filters, may be employed to attenuate undesired signals. Automatic gain control (AGC) mechanisms also have been incorporated into signal chains. An AGC mechanism applies varying gains to a signal that is inputted to the AGC mechanism. As such, desired signals may be amplified sufficiently to allow for meaningful signal processing.
FIG. 1A (Prior Art) illustrates a system 100 for automatically controlling gain in a signal chain. System 100 includes a processing mechanism 140 and an AGC mechanism 180. Processing mechanism 140 includes an input 130, which receives an input signal 101, and an output 150, which outputs a processed signal 160. Input signal 101 comprises a desired signal 110 and an undesired signal 120. AGC mechanism 180 includes an input 170, which receives processed signal 160, and an output 190, which outputs an amplified signal 195.
In some applications, desired signal 110 may be present intermittently in input signal 101. As such, FIG. 1A depicts desired signal 110 with a dashed line. In a voicemail application, for example, a message may include pauses when no voice signals are present.
When desired signal 110 and undesired signal 120 are both present in input signal 101 and have approximately the same order of magnitude, as shown in FIG. 1A, system 100 performs appropriately. More specifically, processing mechanism 140 attenuates undesired signal 120 within input signal 101 so that processed signal 160 outputted by processing mechanism 140 contains a small undesired signal component 162 relative to a desired signal component 161 therein. Processed signal 160 then is presented to input 170 of AGC mechanism 180.
AGC mechanism 180 adjustably applies gain to an input signal at input 170 based on the sensed input signal. In particular, AGC mechanism 180 adjustably applies gain to processed signal 160 based on the sensed magnitude of processed signal 160. For instance, if processed signal 160 is of small magnitude, AGC mechanism 180 applies a relatively large amount of gain to processed signal 160 to ensure that a desired signal component 196 of amplified signal 195 is sufficiently large for use by the associated signal processing application. In contrast, if processed signal 160 is of large magnitude, AGC mechanism 180 applies a relatively small amount of gain to processed signal 160. Based on the adjustably applied gain, AGC mechanism 180 outputs, at output 190, amplified signal 195, which contains desired signal component 196 and undesired signal component 197.
FIG. 1B (Prior Art) shows how input signal 101 is processed when desired signal 110 is not present, or is of small magnitude relative to undesired signal 120. Processing mechanism 140 receives at input 130 input signal 101, which comprises undesired signal 120. Processing mechanism 140 then attenuates undesired signal 120 to produce a processed signal 160 at output 150. Processed signal 160 is received at input 170 of AGC mechanism 180. Because AGC mechanism 180 adjustably applies gain based upon the sensed magnitude of processed signal 160 at input 170, and because desired signal 110 is not substantially present, AGC mechanism 180 applies an increased gain to the relatively small processed signal 160, thereby producing a large amplified signal 195. Amplified signal 195 contains a large undesired signal component 197.
Therefore, implementations such as that shown in FIGS. 1A and 1B do not effectively remove undesired signals when undesired signal components are large relative to desired signal components. Although undesired signals can be attenuated by one or more processing mechanisms, a subsequent AGC mechanism excessively amplifies the attenuated signals. Excessive amplification may impair signal postprocessing mechanisms (not shown) that receive and process amplified signal 195.
Therefore, what is needed is a system and method for automatically controlling gain in a signal chain.